This invention relates to a system having plural signal-carrying channels, such as a scanning array antenna, and more particularly, to a method and an apparatus for fault testing components of the system, as well as for measuring and adjusting starting phases of an array antenna prior to a scan in order to form a focused beam.
Plural channel systems are often constructed with numerous signal-carrying channels arranged in parallel. Examples of such systems are found in electrical communication, including telemetry where different messages are transmitted along parallel channels, apparatus for sound amplification and recording, where a signal is divided among separate spectral passbands for correction of a loudness characteristic, and array antennas, where multiple channels provide various time delays and/or phase shifts for steering a beam of radiation.
In any of the foregoing plural-channel systems, optimum operation of the system is obtained when the design characteristics of the respective channels are maintained. Such characteristics may include preset time delays, preset phase shifts, and preset amplification factors for signals propagating through the channels. A calibration process and equipment are utilized to provide optimum operation.
One area of considerable interest occurs in a microwave landing system (MLS) used for guiding aircraft to a landing on an airport runway. An array antenna is advantageously employed in an MLS for forming a glide slope beam or a localizer beam. The direction and the pattern of the beam are dependent on the phase shifts (and possibly on amplitude shading factors) applied to radiators of the array of individual signal-carrying channels coupled to respective ones of the radiators. MLS phase calibration is employed to measure the insertion phase of each channel of an MLS array. The calibration information is used to adjust the starting phase of each element in the array to compensate for the insertion phase errors resulting from manufacturing processes, tolerances, and component aging. Proper MLS phase calibration results in a well-focused beam with low side lobes and should be performed periodically to correct aging effects. It is advantageous if such calibration could be accomplished automatically, and at sufficiently frequent intervals, so as to correct for any differential phase shifts induced among channels of the system due to thermal expansion, which expansion may occur, by way of example, in a phased array antenna located at an airport runway and illuminated directly by rays of the sun. Calibration of the array antenna is important to ensure that the beam is properly formed and correctly directed for a safe landing by the aircraft.
A problem arises in that the calibration of the foregoing systems has entailed overly complex equipment and time-consuming processes. For example, in the case of a phased array antenna, such processes have employed the injection of testing signals followed by the measurement of in-phase and quadrature components utilizing complex algorithms in computers. This problem is due, in part, to the difficulty of measuring one channel without being "swamped" by all other channels in the array. An attempt to solve this problem by automatically calibrating a system having plural-signal carrying channels is disclosed in U.S. Pat. No. 4,520,361 issued in the name of R. F. Frazita on May 28, 1985, and assigned to Hazeltine Corporation, the assignee herein. In the Frazita patent, a waveguide manifold is employed for extracting microwave signal samples from each of a plurality of radiators of an array antenna, which samples are compared to a reference signal. A continuously incremented phase shift as introduced between a channel under test and the reference signal for a serrodyning of one signal relative to the other signal with a consequent frequency shift between the two signals. Upon mixing the two signals, a beat frequency signal is obtained wherein the phase shift is dependent on component parameters such as insertion phase shift and delay. By measuring the delay in the beat frequency signal relative to a phase shift pattern employed in the serrodyning, a proper value of compensatory phase shift is determined and is inserted as a preset value of phase shift in the channel under test. The procedure is repeated for each of the channels so that differential phase shift can be minimized for improved formation of a transmitted beam of radiation. While the system of the Frazita patent functions properly, it does not solve the foregoing problem completely because the system of the Frazita patent provides the reference signal by means of an additional signal channel, which itself may introduce error in the calibration of the other channels.
It is, therefore, an object of the present invention to provide a new and improved apparatus and method for calibrating a system having a plural signal-carrying channels.
While the invention is ideally suited for use in calibrating numerous types of systems, the invention will be described in the context of calibration apparatus for use with a phased-array antenna of the type used in a microwave landing system (MLS). The physical structures of the components utilized in construction of the invention are such as to permit their coupling to the phased-array antenna with a minimal addition to the complexities of the structure of the antenna itself. Also, electronic circuitry of the invention is operated readily in conjunction with circuitry which operates the MLS.
Individual radiators of the antenna are energized by separate signal-carrying channels, each of which includes a phase shifter, the phase shifters being coupled, in turn, via a power divider to a common transmitter. The phase shifters are individually actuated by command signals supplied by a beam-steering unit wherein individual phase shifts are applied to the signals energizing the respective radiators for shaping and directing a resulting beam of radiation provided by the array of radiators.